Method and apparatus for gas flow measurement

ABSTRACT

A method and apparatus for measuring gas flow are provided. In one embodiment, a calibration circuit for gas control may be utilized to verify and/or calibrate gas flows utilized for backside cooling, process gas delivery, purge gas delivery, cleaning agent delivery, carrier gases delivery and remediation gas delivery, among others.

CROSS-REFERENCE TO OTHER APPLICATIONS

This application is a continuation of a U.S. patent application Ser. No.11/833,623, filed on Aug. 3, 2007, now U.S. Pat. No. 7,743,670, whichclaims the benefit of U.S. Provisional Application No. 60/822,345 filedAug. 14, 2006. Each of the aforementioned patent application is hereinincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to a method andapparatus for measuring gas flow. More specifically, embodiments of theinvention generally relate to a method and apparatus for measuring gasflows that are provided to a semiconductor processing chamber andrelated utilities.

2. Description of the Related Art

Accurate control of gas flows is an important process control attributecritical to many microelectronic device fabrication processes. Providinggas between a substrate and a substrate support in a semiconductorprocessing chamber is a well-known method for improving heat transferbetween the substrate and the substrate support, thereby enhancing theprecision of substrate temperature control and uniformity. Additionally,precise control of process gas flows into the processing chamber arerequired in order to obtain desired processing results, particularly ascritical dimensions and film thicknesses shrink. Furthermore, gases maybe added to processing chamber effluent streams to mitigate theenvironmental impact of substrate processing. Good control of the gasesadded to the effluent stream is necessary to ensure both cost effectiveand proper remediation.

Conventional gas delivery systems used with semiconductor processingchambers generally include a mass gas flow meter (MFC) as the primaryflow regulation device. However, the accuracy of MFC's may be affectedby a plurality of factors that contribute to an uncertainty of theactual gas flow. For example, the accuracy of the MFC will typicallyvary with changes in temperature, line pressure and volume. Deviationfrom the gas flow set point due to MFC inaccuracy may contribute toprocessing defects, poor emission control and inefficient waste ofcostly gases.

Although conventional pressure control systems have proven relativelyreliable, field experience with the existing technology has increasedthe demand for more accurate measurement of flow. For example, poorcontrol of gas flows used in backside substrate cooling applications mayresult in poor substrate temperate control, thereby causing poor filmdeposition or etching results, which cannot be tolerated in nextgeneration circuit designs.

Therefore, there is a need for an improved method and apparatus formeasuring gas flows so that the delivery of gases in a semiconductorprocessing system may be performed with greater confidence and accuracy.

SUMMARY OF THE INVENTION

A method and apparatus for measuring gas flow are provided. In oneembodiment, a calibration circuit for measuring gas flow may be utilizedto verify and/or calibrate gas flows utilized for backside cooling,process gas delivery, purge gas delivery, cleaning agent delivery,carrier gases delivery and remediation gas delivery, among others.

In one embodiment, an apparatus for measuring gas flow in a processingsystem includes a gas source, a diverter valve, an orifice, a regulatingdevice and a sensing circuit. The regulating device is fluidly coupledbetween the gas source and an inlet of the diverter valve. The orificeis fluidly coupled to a first outlet of the diverter valve and hassubstantially the same flow resistance as a processing chamber. Thesensing circuit is configured to receive the flow of gases passingthrough the orifice.

In one embodiment, the sensing circuit utilizes a calibrated volume forreceiving the gas flow. From properties and/or attributes measured fromthe gas in the calibrated volume, the flow rate and/or pressure of thegas entering the sensing circuit may be verified.

In another embodiment, the sensing circuit utilizes a non-calibratedvolume for receiving the gas flow. From changes in the properties and/orattributes measured over time of the gas in the non-calibrated volume,the flow rate and/or pressure of the gas entering the sensing circuitmay be verified.

In another embodiment, the regulating device may be at least one of avapor delivery module, a flow divider, a pressure controller, aregulator or a mass flow controller. In another embodiment, the sensingcircuit may include a tank having a calibrated volume. In anotherembodiment, the sensing circuit may include vibrating member disposed inthe calibrated volume. In another embodiment, the sensing circuit mayinclude a sensor configured to detect at least one of electrical ormagnetic characteristics of gases disposed in the calibrated volume. Inyet another embodiment, the sensing circuit may include a tank supportedby a cantilever.

A method for measuring gas flow in a semiconductor processing system isalso provided. In one embodiment, a method for measuring gas flow in asemiconductor processing system includes setting a gas flow with a flowcontrol device, flowing the gas from the flow control device through anorifice having substantially the same flow resistance as a processingchamber into a sensing circuit, and comparing a flow determined usingthe sensing circuit with the setting of the flow control device.

In another embodiment, the method may include sampling a characteristicof a gas present in the sensing circuit until an endpoint is reached. Inanother embodiment, the method may include sampling until a confidencelimit is reached. In another embodiment, the method may include samplinguntil data converges within a predetermined range. In yet anotherembodiment, the method may include sampling at a frequency rate of lessthan about 5 millisecond.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention may be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a simplified schematic of a conventional semiconductorprocessing chamber and gas delivery system having a calibration circuitof the present invention; and

FIGS. 2-9 are simplified schematics of a calibration circuit havingvarious embodiments of a sensing circuit.

To facilitate understanding, identical reference numerals have beenused, wherever possible, to designate identical elements that are commonto the figures. It is contemplated that features of one embodiment maybe beneficially incorporated in other embodiments without furtherrecitation.

DETAILED DESCRIPTION

FIG. 1 depicts a simplified schematic of a substrate processing systemhaving one embodiment of a gas delivery system 140 of the presentinvention coupled to an exemplary a semiconductor processing chamber120. The processing chamber 120 may be configured to perform chemicalvapor deposition (CVD), physical vapor deposition (PVD), etch chamber,ion implant, thermal processing, ashing, degassing, orientation or othervacuum processing techniques.

The processing chamber 120 includes a substrate support 124 disposed ina chamber body 122. The substrate support 124 generally supports asubstrate 126 during processing. The substrate support 124 generallyincludes a passage formed therethrough for delivering a heat transfergas (hereinafter referred to as backside gas) to an area 118 definedbetween the substrate 126 and substrate support 124. The size of thearea 118 has been exaggerated in FIG. 1 for clarity. Examples of commonbackside gases include helium, nitrogen and argon.

The chamber body 122 generally includes at least one process gas inlet128 and a pumping port 134. The process gas inlet 128 generally providesprocess and optionally other gases to the interior volume of theprocessing chamber 120 to facilitate substrate processing, as isconventionally known. The gases entering the chamber body 122 may bedistributed across the substrate 126 by a gas distribution plate, orshowerhead 130.

The pumping port 134 is formed in the chamber body 122. The pumping port134 is generally coupled to a pumping system that controls the chamberpressure and removes processing by-products from the interior volume ofthe chamber body 122. The pumping system typically includes one or morevacuum pumps and throttle valves, which are not shown.

A treatment gas port 144 may be provided to deliver remediation gasesinto a conduit 160 carrying the effluent stream exiting the chamber body122 via the pumping port 134. For example, gases may be provided toreact and/or absorb hazardous reaction by-products, excess process gasesor gaseous chamber waste to facilitate removal and/or recovery ofcertain materials from the effluent stream.

A purge port 132 may also be provided in the chamber body 122. Inertgases may be provided through the purge port 132 into the processingchamber 120 to prevent process gases and/or process by-products fromentering certain regions of the chamber 120. Examples of purge gasesinclude nitrogen and helium.

Gases are generally provided to the inlet port 128, the area 118, thepurge port 132 and the treatment gas port 144 from one or more gasdelivery circuits. Each gas delivery circuit generally includes amechanism for the precise control of the gases flowing therethrough, andat least one of which, may be configured as the gas delivery system 140of the present invention. For the sake of brevity, one gas deliverysystem 140 is shown coupled to gas delivery lines 112, 114, 116, 138respectively routed to the inlet port 128, the area 118, the purge port132 and the treatment gas port 144. In practice, each line 112, 114,116, 138 may be respectively coupled to dedicated, separate circuit gasdelivery systems 140.

In one embodiment, the gas delivery system 140 includes a gas source102, a mass flow meter (MFC) 142, a diverter valve 106 and a calibrationcircuit 104. The diverter valve 106 selectively directs flow from thegas source 102 to the calibration circuit 104 or one of the lines 112,114, 116, 138 through a conduit 110. The MFC 142 is disposed between thegas source 102 and the diverter valve 106. The MFC 142 is generallyutilized to monitor and control the flow from the gas source 102 intoeither the calibration circuit 104 or conduit 110 coupling the gasdelivery system 140 to the processing chamber 120.

The calibration circuit 104 is configured to accurately measure gasflow. The calibration circuit 104 includes an orifice 108 and a sensingcircuit 146. The orifice 108 is disposed between the sensing circuit 146and the diverter valve 106. The orifice 108 may be sized such that therestriction maintains a chocked flow condition. In one embodiment, thesize of the orifice is selected to simulate the restriction of theactual processing chamber 120. This creates conditions similar to theMFC 142 flowing into the processing chamber 120 under which to performflow verifications using the calibration circuit 104, while notrequiring flow into the actual processing chamber 120. The orifice maybe determined by experimentation, empirical analysis or by othersuitable method. In one embodiment, the orifice 108 may be determined bymeasuring the pressure downstream of the orifice and adjusting theorifice size until a desired pressure is realized. In anotherembodiment, the size of the orifice may be selected to be different thanthe restriction of the actual processing chamber 120, as long as theflow is maintained in a chocked flow condition.

The orifice 108 is sized to create a critical flow (e.g., chocked flow)condition into the sensing circuit 146. Critical flow into the sensingcircuit 146 means that the flow is determined by the mass flow rate andaperture size of the orifice 108. The flow upstream of the orifice 108(e.g., at the MFC 142) is constant and unaffected by pressurefluctuations, thus the upstream volume need not be considered duringflow calculations.

The orifice 108 may be a fixed or variable restriction. In oneembodiment, the orifice 108 may be a machined aperture. In anotherembodiment, the orifice 108 may be adjustable, such as a needle valve.

FIG. 2 is a schematic diagram of one embodiment of the sensing circuit146. The sensing circuit 146 generally includes a tank 202 having avibrating member 204 disposed therein. A sensor 206 is interfaced withthe vibrating member 204 in a manner suitable for providing a processor208 with a metric indicative of the frequency of oscillation of thevibrating member 204, which may be correlated to the density of gaswithin the tank 202. In one embodiment, the sensor 206 is anaccelerometer or other suitable sensor.

In one embodiment, the tank 202 has a known or predetermined volume. Gasflowing into sensing circuit 146 through the orifice 108 from the gassource 102 will increase the pressure inside the tank 202, and thus thedensity of the gas the tank 202. Since the volume of the tank 202 isknown, the frequency of the vibrating member 204 may be correlated tothe mass of the gas within the tank 202. The change in the frequency ofoscillation of the vibrating member 204, given the known tank volume,provides information related to the density change within the tank 202,which is also related to the mass flow rate through the MFC 142. Thus,the frequency of the vibrating member may be utilized to verify and/orcalibrate the flow through the MFC 142.

In another embodiment, the volume of the tank 204 may not be known. Inthis embodiment, the change in frequency of the oscillating member 204may be used to verify and/or calibrate the flow rate through the MFC142.

FIG. 3 is a schematic diagram of another embodiment of a sensing circuit300. The sensing circuit 300 generally includes a tank 202 having avibrating member 204 and a secondary member 302 disposed therein. Thesecondary member 302 is positioned opposite the vibrating member 204. Afirst sensor 206 is interfaced with the vibrating member 204 in a mannersuitable for providing a processor 208 with a metric indicative of thefrequency of oscillation of the vibrating member 204. A second sensor304 is interfaced with the secondary member 302 in a manner suitable forproviding the processor 208 with a metric indicative of the frequency ofoscillation of the secondary member 302.

The vibrating member 204 may be driven at a constant frequency. Energyfrom the vibrating member 204 is transferred through the gas disposed inthe tank 202 and causes the second member 302 to oscillate at a constantfrequency. The oscillation of the second member 302 will have a phaseshift and amplitude of vibration different than that of the vibratingmember 204. These quantities may be measured by the sensors 206, 304 andrelated to a change in pressure in the tank, which over time relates toa mass flow rate which may be utilized to verify and/or calibrate theflow rate through the MFC 142.

FIG. 4 is a schematic diagram of another embodiment of a sensing circuit400. The sensing circuit 400 generally includes a tank 402 mounted in acantilevered orientation from a surface 404. The mass of gas in the tank402 is related to the deflection of the cantilevered tank, which may bemeasured by a sensor 406. The sensor 406 may be a strain gauge or adistance measuring device, such as an LVDT. As gas flows into the tank402, the pressure and density of the gas in the tank 402 will increase,thereby causing a change in the orientation of the tank 402 which iscorrelated to the additional mass of gas that has been added to the tank402. The change in tank orientation due to the change in the mass of gasin the tank may be measured by the sensor 406. Thus, the informationfrom the sensor 406 may be utilized to verify and/or calibrate the MFC142.

FIG. 5 is a schematic diagram of another embodiment of a sensing circuit500. The sensing circuit 500 generally includes a tank 502, adisplacement device 504 and a sensor 506. The tank 502 has a calibratedvolume. The displacement device 504 may be actuated to disturb the tank502 such as to cause the tank 502 to oscillate. The displacement device504 may be a transducer, actuator, or other suitable vibrationgenerating device. The sensor 506, which may be an accelerometer orother suitable detector, is interfaced with the tank 502 to provide aprocessor 208 with a metric indicative of the frequency of the tankoscillation. As the mass of gas in the tank 502 increases, the frequencyof oscillation will change in a predictable manner which is indicativeof a mass flow rate into the tank 502. Thus, the information obtained bythe sensor 506 may be utilized to verify the flow rate through the MFC142.

In another embodiment, the wall of the tank 502 may be perturbed by thedisplacement device 504 such that it vibrates. As the pressure insidethe tank 502 changes, the stress on the wall will change and thefrequency of the vibration will change predictably. The vibration may bemeasured by the sensor 506, and because the volume is known, thepressure change over time may be related to the mass flow rate enteringthe sensing circuit 500 and may be utilized to verify and/or calibratethe flow rate through the MFC 142.

FIG. 6 is a schematic diagram of another embodiment of a sensing circuit600. The sensing circuit 600 generally includes a tank 602, a signalgenerator 604 and a sensor 606. The tank 602 has a calibrated volume.The signal generator 604 and the sensor 606 may be mounted inside oroutside the tank 602.

In one embodiment, the signal generator 604 is configured to generate anacoustic pulse inside the calibrated volume of the tank 602. The localspeed of the acoustic pulse is related to the density and temperature ofthe medium (e.g., the gas within the tank). The speed of the acousticpulse may be measured by the sensor 606, and related to the density ofthe gas within the tank 602. As the volume of the tank 602 is known, themass of the gas within the tank 602 may be determined using the sensorinformation and utilized to verify and/or calibrate the flow ratethrough the MFC 142.

In another embodiment, the signal generator 604 may provide an RF signalor other electromagnetic pulse in the tank 602 to measure the density ofgas within the tank. Characteristics of these signals vary predictablywith pressure and the sensor 606 may be utilized to provide a metricindicative of at least one of the signal characteristics. The change ofthe measured characteristics over time may be correlated to a mass flowrate into the sensing circuit 600, and utilized to verify and/orcalibrate the flow rate through the MFC 142.

In another embodiment, the sensor 606 may be configured to detectchanges in at least one of electrical or magnetic characteristics of thegas within the tank 602. The electrical or magnetic characteristicschange predictably with pressure and may be measured by the sensor 606.The metric of change in the electrical or magnetic characteristics ofthe gas provided by the sensor 606 may be used to determine flow ratethrough the circuit 600. The pressure change over time relates to a massflow rate in the known volume of the tank 602, and as such, may beutilized to verify and/or calibrate the flow rate through the MFC 142.

FIG. 7 is a schematic diagram of another embodiment of a sensing circuit700. The sensing circuit 700 generally includes a tank 702 having apiston 704 disposed therein. The piston 704 has a known weight andsurface area. The piston 704 will be displaced against an opposingmember 706 relative to the pressure within the tank 702. The opposingmember 706 may be a spring and/or sealed volume of gas. The force neededto move the piston 704 may be resolved from the mass and surface area ofthe piston 704, the spring force of the opposing member 706, along withthe pressure above the piston 704, which can either be controlled orknown as a function of piston displacement. In one embodiment, a sensor708 is interface with the piston 704 to directly determine the forceacting upon the piston. In another embodiment, the sensor 708 may beconfigured to determine the displacement of the piston 704. The force isrelated to the pressure by the surface area of the piston 704, and thepressure change over time in the known volume may be related to the massflow rate into the tank 704 and utilized to verify and/or calibrate theflow rate through the MFC 142.

FIG. 8 is a schematic diagram of another embodiment of a sensing circuit800. The sensing circuit 800 generally includes a tank 802 having apiston 704 disposed therein. Flow from the orifice 108 is provided intothe tank 802 through first and second inlets 810, 812 respectivelypositioned above and below the piston 704. The piston 704 has knowncharacteristics and will be displaced proportionately to a ratio offlows through the inlets 810, 812. At least one sensor is utilized todetermine the relative displacement of the piston 704. In the embodimentdepicted in FIG. 8, sensors 804, 806 are utilized to determine thedisplacement of the piston 704 which may be correlated to the flowthrough the MFC 142.

FIG. 9 is a schematic diagram of another embodiment of a sensing circuit900. The sensing circuit 900 generally includes a plurality of tanks(shown as tanks 902, 904, 906) coupled to the orifice 108 through aselector valve 908. Each tank 902, 904, 906 has a different calibratedvolume for use with different ranges of flow rates. For example, tank902 may have a small volume, tank 904 may have an intermediate volume,while tank 906 may have a large volume. The valve 908 is utilized todirect the flow in the circuit 900 to a tank having a volumecommensurate flow rate so that good data resolution may be obtained overa reasonable sampling period. For example, the smaller tank 902 may beutilized to achieve to achieve greater time resolution for a givenpressure rise over a given period of time when low flow rates are beingmeasured. Thus, at low flow rates, the smaller tank 902 facilitateobtaining a rapid pressure rise over time, which provides data sethaving good resolution over a short sample period. Conversely, thelarger tank 908 may be utilized to obtain a pressure rise over time thatis not too rapid, thereby providing data having good resolution at highflow rates. Moreover, the larger tank 908 allows the data sample tooccur over a longer period of time since the tank holds more volumerelative to the small tank 902, which may be filled at high flow ratesbefore a complete data set is obtained. Data relating to the pressurerise may be obtained using any of the techniques described above, orother suitable alternative.

Multiple data samples of the pressure rise may be utilized to improvethe accuracy of the flow calculation, thereby providing greaterconfidence of the actual flow rate of the MFC 142. Each of the sensingcircuits described above may include a bypass loop and dump line whichallows the tank to be rapidly emptied and refilled, thereby facilitatingthe rapid acquisition of additional samples. Data samples are obtainedat a sufficient rate to obtain a statistically valid sample populationof data points in a reasonable period of time. In one embodiment, thefrequency rate of sampling is less than about 5 milliseconds. Thisallows a large data set to be obtained over a shorter test duration,thereby increasing the accuracy of the data while allowing anappropriate endpoint of the test to be rapidly identified.

In one embodiment, the combined measurement error of thesensors/equipment utilized to obtain the data samples may be analyzed todetermine their effect on the overall calculation. This information maybe utilized to determine and/or adjust the confidence limits.

In another embodiment, the combined measurement error of thesensors/equipment utilized to obtain the data samples may be utilized tosimulate a random error distribution in the measured data. A simulatederror is then added to each data sample. The number of samples needed tocancel out the effects of the randomly added error may be calculated andutilized as a test end point so that the accurate calculations arerealized within the shortest test duration

An exemplary bypass loop 250 and dump line 252 are shown in theembodiment depicted in FIG. 2. The flow from the orifice 108 initiallygoes into the tank 202 and the pressure (density and/or mass) ismeasured. The flow through the sensing circuit 200 is then diverted by avalve 256 through the bypass loop 250 while a second valve 258 is openedto empty the tank 202 through the dump line 252. The dump line 252 maybe coupled to an exhaust to expedite the removal of gases from the tank202. Once the tank 202 is sufficiently emptied, the second valve 258 isclosed and the flow from the orifice 108 is then directed by the valve256 back to the tank 202 so that a subsequent sample may be obtained.This process may be repeated multiple times to obtain a data set thatprovides an accurate measurement of the flow through the MFC 142.

The processor 208 receiving the data set may use statistical convergencetechniques and/or classical robust statistics to determine anappropriate endpoint of the flow verification/calibration. For example,the sampling may be terminated once a suitable convergence calculatedbased on the known accuracy and repeatability of the measurement devicesof the sensing circuit is reached. The sampling endpoint mayalternatively be determined dynamically by continuously calculatingflows and tracking the convergence toward a mean value.

The desired level of convergence may be a predetermined level or bedetermined dynamically using a confidence limit. Once the test hasreached a specified confidence level, the test will terminate. Onemethod of ending the test is to use the known error levels of themeasurement devices and use them to calculate the number of samplesneeded for convergence. Using this predictive method the verificationwill automatically end once that number of samples has been taken.Another method to determine the endpoint is to continuously recalculatethe flow and monitor its convergence toward a mean value. As the testruns, every combination of samples collected may be used to calculatethe instantaneous flow. When the calculated flows converge to a desiredlevel, the test will terminate. The use of multivariate models andstatistics to model measurement errors and their effects on the overallsystem may be used to increase the accuracy of the calculations. Themodels will show the interaction of different parameters and aid in theselection of optimal parameters.

Multiple samples of the beginning and ending pressures and/or densitiesmay be used to increase accuracy. Measurement error on both readingswill be decreased by averaging multiple samples of each reading, thusmaking the actual pressure delta for the test more accurate.

Multivariate models and statistics may be used to model the error of theindividual measurements and their effects on the overall system error.These models may be used to determine optimal parameters and systemlimits. Combinations of the previous techniques may be used to furtherincrease the accuracy of the flow rate calculation.

In operation, flow is determined using standard rate of rise techniqueswithin the tank. The orifice at the inlet of the tank is sized to createsonic flow entering the tank. The flow into the tank is then relatedonly to the flow from the MFC and the size of the orifice. The tankpressure will have no effect on the MFC, and as such, will allow theflow to remain constant. In addition, the sonic condition at the orificeprevents the upstream pressure from changing, and as such, the mass ofgas in the gas line upstream of the orifice remains constant. Under thiscondition the gas line volume upstream is not utilized in flowcalculations, thereby eliminating the need for an upstream volumecalculation and further reducing uncertainty in overall flowcalculation.

Since the orifice at the tank inlet simulates chamber conditions byemulating a restriction similar to a chamber injection, the MFC can becalibrated under simulated chamber conditions without requiring theactual chamber to be physically present, for example, during bench orpre-installation testing. Alternatively, the flow through the MFC may beverified and/or calibrated using the calibration circuit as desired oncethe chamber is in operation, such as periodic test performed prior torunning a new lot of substrates.

It is also contemplated that the calibration circuit may be utilized toverify and/or calibrate flow control devices other than MFC's. Forexample, the calibration circuit may be utilized to verify and/orcalibrate flow rates (density and/or pressures) from vapor deliverymodules, flow dividers, pressure controllers and regulators, among otherflow control devices.

Thus, gas delivery systems having calibration circuit thatadvantageously enable characterization of the MFC utilized to providegases to a processing system. The innovative calibration circuit may beutilized to measure, verify and/or calibrate gas flows utilized forbackside cooling, process gas delivery, purge gas delivery, cleaningagent delivery, carrier gases delivery and remediation gas delivery,among others. The accuracy and sampling time of the gas flow control hasbeen improved over the state of the art, thereby enabling cost effectiveand robust processing of next generation devices.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. Apparatus for verifying a gas flow in a processing system having aprocessing chamber, comprising: a gas source; an orifice having a flowresistance substantially the same as a flow resistance of the processingchamber; a diverter valve having an inlet, a first outlet and a secondoutlet, wherein the first outlet is coupled to the orifice and thesecond outlet is coupled to the processing chamber; a regulating devicefluidly coupled between the gas source and the inlet of the divertervalve; a sensing circuit coupled to the orifice, wherein the sensingcircuit receives a flow passing through the orifice to verify the flowthrough the regulating device.
 2. The apparatus of claim 1, wherein thesensing circuit further comprises a tank which receives the flow passingthrough the orifice.
 3. The apparatus for claim 2, wherein the tank hasa calibrated volume for receiving the flow passing through the orifice.4. The apparatus for claim 3, wherein the sensing circuit furthercomprises: a vibrating member disposed in the calibrated volume of thetank.
 5. The apparatus for claim 4, wherein the sensing circuit furthercomprises: a sensor which detects at least one of electrical or magneticcharacteristics of gases disposed in the calibrated volume.
 6. Theapparatus for claim 3, wherein the sensing circuit further comprises: adisplacement device attached to the tank, wherein the displacementdevice causes the tank to oscillate; and a sensor interfaced with thetank.
 7. The apparatus for claim 3, wherein the sensing circuit furthercomprises: a signal generator which generates an acoustic pulse insidethe calibrated volume of the tank; and a sensor which measures theacoustic pulse generated by the signal generator.
 8. The apparatus forclaim 2, wherein the tank has a non-calibrated volume for receiving theflow passing through the orifice.
 9. The apparatus for claim 8, whereinthe sensing circuit further comprises: a cantilever supporting the tank;and a sensor which measures measure a change in orientation of the tank.10. The apparatus for claim 8, wherein the sensing circuit furthercomprises: a piston disposed in the tank; and a sensor which determinesa displacement of the piston.
 11. The apparatus of claim 1, wherein thesensing circuit further comprises: a plurality of tanks, wherein each ofthe plurality of tanks has a calibrated volume receiving the flowpassing through the orifice, and the calibrated volume of the pluralityof tanks is different from one another; and a selector valve coupledbetween the orifice and the plurality of tanks.
 12. The apparatus forclaim 2, wherein the sensing circuit further comprises: a by-pass loop;and a valve diverting the flow from the orifice between the tank and theby-pass loop.
 13. The apparatus of claim 1, wherein the orifice is avariable restriction having an adjustable flow resistance.
 14. A methodfor calibrating gas flow in a semiconductor processing system,comprising: setting a gas flow from a gas source towards a sensingcircuit with a flow control device; maintaining the gas flow in achocked flow condition by flowing the gas flow from the flow controldevice to an orifice having a flow resistance substantially the same asa flow resistance of a processing chamber; and determining a property ofthe gas flow by flowing the gas flow from the orifice into the sensingcircuit which calibrates the gas flow set by the flow control device.15. The method of claim 14, wherein determining the property comprises:flowing the gas into a tank of the sensing circuit, wherein the tank hasa calibrated volume; sensing at least one property and/or attribute ofthe gas in the calibrated volume; and resolving a flow rate and/orpressure of the gas entering the sensing circuit from the sensedproperty and/or attribute.
 16. The method of claim 14, whereindetermining the property comprises: flowing the gas into a tank of thesensing circuit, wherein the tank has a non-calibrated volume; sensingat least one property and/or attribute of the gas in the non-calibratedvolume; and resolving a flow rate and/or pressure of the gas enteringthe sensing circuit from the sensed property and/or attribute.
 17. Themethod of claim 14, further comprising comparing a flow rate determinedusing the sensing circuit with the setting of the flow control device.18. The method of claim 14, further comprising adjusting the orifice sothat the flow resistance of the orifice is substantially the same as theflow resistance of the processing chamber.
 19. The method of claim 18,wherein adjusting the orifice comprises: measuring the pressuredownstream of the orifice; and adjusting the size of the orifice until adesired pressure is realized.
 20. Apparatus for verifying a gas flow ina processing system having a processing chamber, comprising: a gassource; a regulating device fluidly coupled to the gas source; a sensingcircuit receiving a flow passing through the regulating device to verifythe flow through the regulating device; a diverter valve having aninlet, a first outlet and a second outlet, wherein the inlet is coupledto the flow regulating device, the first outlet is coupled to thesensing circuit, the second outlet is coupled to the processing chamber,and the diverter valve selectively directs the flow from the regulatingdevice to the sensing circuit or the processing chamber; and an orificedisposed between the diverter valve and the sensing circuit, wherein theorifice is sized to create a critical flow condition, and the criticalflow condition allows the sensing circuit to verify the flow from theregulating device without requiring a flow into the processing chamber.21. The apparatus of claim 20, wherein the orifice is a fixedrestriction having a flow resistance substantially similar to a flowresistance of the processing chamber.
 22. The apparatus of claim 20,wherein the orifice is a variable restriction having an adjustable flowresistance.